$ D = \left[\begin{array}{rrr}-1 & -2 & 0 \\ 5 & 3 & 4\end{array}\right]$ $ A = \left[\begin{array}{rr}5 & 2 \\ 2 & 4 \\ 5 & 1\end{array}\right]$ What is $ D A$ ?
Answer: Because $ D$ has dimensions $(2\times3)$ and $ A$ has dimensions $(3\times2)$ , the answer matrix will have dimensions $(2\times2)$ $ D A = \left[\begin{array}{rrr}{-1} & {-2} & {0} \\ {5} & {3} & {4}\end{array}\right] \left[\begin{array}{rr}{5} & \color{#DF0030}{2} \\ {2} & \color{#DF0030}{4} \\ {5} & \color{#DF0030}{1}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ D$ , with the corresponding elements in column $j$ of the second matrix, $ A$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ D$ with the first element in ${\text{column }1}$ of $ A$ , then multiply the second element in ${\text{row }1}$ of $ D$ with the second element in ${\text{column }1}$ of $ A$ , and so on. Add the products together. $ \left[\begin{array}{rr}{-1}\cdot{5}+{-2}\cdot{2}+{0}\cdot{5} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ D$ with the corresponding elements in ${\text{column }1}$ of $ A$ and add the products together. $ \left[\begin{array}{rr}{-1}\cdot{5}+{-2}\cdot{2}+{0}\cdot{5} & ? \\ {5}\cdot{5}+{3}\cdot{2}+{4}\cdot{5} & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ D$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ A$ and add the products together. $ \left[\begin{array}{rr}{-1}\cdot{5}+{-2}\cdot{2}+{0}\cdot{5} & {-1}\cdot\color{#DF0030}{2}+{-2}\cdot\color{#DF0030}{4}+{0}\cdot\color{#DF0030}{1} \\ {5}\cdot{5}+{3}\cdot{2}+{4}\cdot{5} & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{-1}\cdot{5}+{-2}\cdot{2}+{0}\cdot{5} & {-1}\cdot\color{#DF0030}{2}+{-2}\cdot\color{#DF0030}{4}+{0}\cdot\color{#DF0030}{1} \\ {5}\cdot{5}+{3}\cdot{2}+{4}\cdot{5} & {5}\cdot\color{#DF0030}{2}+{3}\cdot\color{#DF0030}{4}+{4}\cdot\color{#DF0030}{1}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}-9 & -10 \\ 51 & 26\end{array}\right] $